Underwater wireless communication outperforms cable-based communication with better maneuverability and simplicity of deployment, in which underwater wireless optical communication (UWOC) has attracted considerable interest due to its merits of low cost, large bandwidth, low latency, and high flexibility compared to mature acoustic communication. Furthermore, UWOC shows the prospect of enhancing the performance of current underwater wireless sensor networks (UWSNs). In this paper, dual-polarization multiplexing is employed to double the transmission capacity in the proposed LED-based UWOC system. Furthermore, Volterra series-based nonlinear equalization is utilized to mitigate the nonlinear impairment and thus further improve the system performance. The effectiveness of single-carrier nonlinear equalization is experimentally verified in the LED-based dual-polarization multiplexing UWOC system. The results show that compared with linear equalization, > 3dB signal-to-noise ratio (SNR) gain can be achieved by the nonlinear equalization in the dual-polarization UWOC system. Compared with the single polarization system, the transmission capacity approximately doubles in the dual-polarization multiplexing UWOC system, and the cross talk between x and y polarization is also analyzed.